After more than 7 years i still use Mercury mail server. As a private consultant, i recommend it. It is a perfect solution for small and medium networks. Easily works for 20 to 300 users. Easy to install (it usually takes 90 seconds, measured) to have a mail server working. I have made more the 30 installations all working. No per user cost, completely compatible SMTP/POP3, works with standard antivirus. FULL MAIL CONTROL, mail-in mail-out control, strict relay control makes a relay proof mail server. No WEB mail yet, but everything else works perfect. The best is the simplicity, process windows ideal for learning purposes, and backup/restore is so simple as to copy a directory. Full server restore in under 3 minutes. Works with windows 95-up. And works from Pentium 166/64 MB. As i mentioned this is for a personal - workgroup solution. In this scenario I did not find problems yet.
Price: Free review details